Procalcitonin can help differentiate bacterial from viral CAP, but no specific cutoff found

For distinguishing any type of bacterial community-acquired pneumonia (CAP) from viral CAP, a procalcitonin threshold of 0.1 ng/mL produced a sensitivity of 80.9% and a specificity of 51.6%.

Procalcitonin values demonstrated accuracy in differentiating viral from bacterial community-acquired pneumonia (CAP) in a recent study, but not at clear-cut thresholds.

Researchers used data from 1,735 adults hospitalized with CAP (median age, 57 years) who were enrolled in the CDC Etiology of Pneumonia in the Community study from January 2010 through June 2012. Participants had procalcitonin measurements and underwent systematic bacterial and viral pathogen testing.

Based on microbiology test results, researchers categorized patients into five pathogen groups: typical bacteria (169 patients [10%]), atypical bacteria (67 [4%]), virus (409 [24%]), mycobacteria/fungus (15 [1%]), and no pathogen detected (1,075 [62%]). They conducted three analyses to assess the accuracy of procalcitonin in identifying bacterial CAP, comparing 1) patients with any bacterial pathogen to those with a viral pathogen, 2) patients with typical bacteria to those with atypical bacteria or viruses, and 3) patients with any bacterial pathogen to those who did not have bacteria detected.

Results were published online on April 12 by Clinical Infectious Diseases.

Researchers used the following procalcitonin values, which have been described in the literature as thresholds for identifying bacterial CAP: <0.1 ng/mL, 0.1 to 0.24 ng/mL, 0.25 to 0.49 ng/mL, and ≥0.5 ng/mL.

Median procalcitonin was lower in the viral group compared to the atypical bacterial group (0.09 ng/mL vs. 0.20 ng/mL, P=0.05) and typical bacterial group (2.5 ng/mL, P<0.01). Typical bacteria were more prevalent in patients with higher procalcitonin concentrations (3% among those with <0.1 ng/mL vs. 21% among those with ≥0.5 ng/mL). Among patients with typical bacteria, 23.1% had procalcitonin <0.25 ng/mL, and 12.4% had procalcitonin <0.1 ng/mL.

For distinguishing any bacterial CAP from viral CAP, a procalcitonin threshold of ≥0.1 ng/mL produced a sensitivity of 80.9% (95% CI, 75.3% to 85.7%) and a specificity of 51.6% (95% CI, 46.6% to 56.5%). For distinguishing typical bacterial CAP from viral and atypical CAP, the same threshold produced a sensitivity of 87.6% (95% CI, 79.6% to 90.7%) and a specificity of 49.3% (95% CI, 44.8% to 54.0%). For bacterial CAP versus nonbacterial CAP, sensitivity was 80.9% (95% CI, 75.3% to 85.7%) and specificity was 46.2% (95% CI, 43.7% to 48.8%).

The study authors noted limitations, such as how almost a quarter of patients from the CDC study were excluded because procalcitonin measurements were not available and that, for those with values, the biomarker was only measured at the time of admission. They wrote that procalcitonin concentrations “could be a useful adjunct in the etiologic assessment of patients hospitalized with CAP,” but added that “clinicians cannot rely on procalcitonin alone to guide antibiotic decisions.”

Procalcitonin appears to correlate with viral or bacterial etiologies of acute respiratory tract infections, “but not terribly well,” according to an accompanying editorial. However, the biomarker can likely be used to help guide clinical management, the editorialists stated. “The challenge going forward is learning how to reconcile host and pathogen-based diagnostics to gain a comprehensive understanding of the patient's disease,” they wrote.